Mobile-to-mobile calls (MMCs) typically require today that a speech signal is digitally encoded in the originating mobile user device, sent over a first air interface, transcoded into PCM encoded speech by a first transcoder unit in a fixed network, carried over the fixed network, transcoded again by a second transcoder unit, sent over a second air interface and is finally decoded in the terminating mobile user device. The speech signal is thus typically transcoded from a compressed speech codec adapted to the first air interface into an intermediate representation, typically into PCM (Pulse Code Modulation) encoded speech according to G.711, and is then transcoded again into a speech codec adapted to the second air interface, which may be the same or another codec as used over the first air interface. The term “codec” refers to a coding-decoding mechanism for performing encoding and decoding on a digital data stream.
Tandem-free operation (TFO) as defined by the 3GPP (3rd Generation Partnership Project) in its TS (Technical Specification) 28.062 provides a mechanism that avoids the two transcoding steps in MMCs if the speech codecs used over the air interfaces are compatible to each other. To this end, the TS 28.062 defines a TFO protocol for in-band signalling between the two transcoder units involved in the MMC. The transcoder units may for example include a TRAU (Transcoding Rate Adaptation Unit) in a GSM network or a MGW (Media Gateway) in an UMTS network.
TFO will be established in case the speech codecs used over the air interfaces are identical or at least compatible with each other. In case TFO is operational, each transcoder unit receives speech frames via its air interface and forwards them without transcoding (e.g., embedded into a PCM link in the network) to its TFO partner, the distant or remote transcoder unit, which then forwards the received speech frames without transcoding via its air interface.
Any in-path equipment (IPE) in the call path between the two transcoder units, which may normally modify the speech signal, has to be removed from the path (or made transparent), because it may destroy the TFO control signalling (i.e. the transparent TFO channel) which would lead to the abortion of a TFO procedure. Such in-path equipment may comprise for example echo cancellers or level shifters, cf. TS 28.062, Annex B.
For establishing TFO, the two transcoder units have to initiate a TFO in-band negotiation procedure. Each of the transcoder units indicates its locally used speech codec (termed “Local Used Codec”, LUC, in the TS 28.062) to the TFO partner. If the codecs are at least compatible to each other, TFO will be established in the (PCM) call path between the two transcoder units. The establishment procedure comprises exchanging transparent mode messages, e.g. the TFO_TRANS(n) messages according to the TS 28.062, which indicate to the IPE along the path that subsequent speech frames will be exchanged as well as TFO control bits and TFO messages according to the TFO in-band signalling protocol which will need to be transparently exchanged via the PCM link within the fixed network. The IPE will from then on pass the respective bits in the PCM frames transparently. TFO is then in the state “OPERATION”, and TFO frames, which include speech frames as received via the air interfaces, may be exchanged between the TFO partners.
When TFO is in the OPERATION state, TFO codec lists (TCLs) may be exchanged for (further) codec optimization. The TCLs may be exchanged via configuration frames, e.g. the Generic Configuration Frames (GCFs) defined in TS 28.062, Annex H, whose transmission mechanism differs from the TFO signalling messages according to the TFO in-band signalling protocol. The GCFs may be sent as replacement of the TFO speech frames. This does not degrade the speech quality during already established TFO, as such a replacement is performed only sporadically. The instantaneous (short term) data rate is as high as the data rate for TFO frames, e.g. 8 kbit/s (i.e. 160 bit/20 milliseconds) or 16 kbit/s (i.e. 320 bit/20 milliseconds). Alternatively, the TCLs may be transmitted without TFO being established (e.g. in the TFO state MISMATCH) by TFO signalling messages such as the TFO_REQL or TFO_ACK_L messages, and thus are conveyed with an instantaneous data rate of only less than 500 bits per second only (to not distort the speech quality).
If it is determined that the LUCs of the two transcoder units are incompatible to each other, TFO cannot be established. In that case, no transparent mode messages are exchanged. TFO signalling messages may be exchanged nonetheless, including the TCL specifying each codec type available for the respective TFO interface. However, this procedure is slow (a single TFO_REQ_L signalling message including a complete TCL takes at least 180 milliseconds and may take on the average 300 milliseconds) compared to TFO establishment in case of matching LUCs, which takes of order several ten milliseconds only. Further, the codec types as exchanged via the TFO inband signalling mechanism are represented as a bit vector, in which one specific bit signals the support of a certain codec type. This mechanism does not allow the signalling of a preference of particular codec types nor does it allow to specify detailed configuration information for a particular codec.
A technique is needed for controlling establishment of a tandem-free transcoder operation in case the locally used codecs of both sides are not compatible to each other, which technique is fast, reliable and does not have an undesirable impact on the speech quality.